The invention relates to soldering and tinning operations carried out using machines comprising a bath of liquid solder, wherein the bath is either a xe2x80x9cstatic bathxe2x80x9d or is set in motion as in the machines known as xe2x80x9cwave solderingxe2x80x9d machines.
These machines are used in particular for soldering electronic components on a support such as an electronic circuit, or for tinning the terminals of electronic components.
Description of the Related Art
Wave soldering machines are designed in such a manner that the circuits to be soldered (or the pieces to be tinned) are brought into contact with one or more waves of liquid solder generated by pumping a solder bath residing in a tank through a nozzle.
The pieces are generally fluxed beforehand in a zone upstream from the machine, primarily to deoxidize the metal surfaces in order to facilitate their subsequent wetting by the solder. The fluxing operation is followed by a preheating operation which is carried out both in order to activate the flux previously deposited on the circuit and in order to preheat the circuits and components prior to their arrival in the hot soldering zone.
The geometric configuration of the nozzle determines the shape of the solder wave. Wave soldering machines usually have two waves, a first so-called xe2x80x9cturbulentxe2x80x9d wave and a second so-called xe2x80x9claminarxe2x80x9d wave that presents a relatively large flat upper surface.
In the absence of pieces to be soldered or tinned in the machine, the liquid solder in this laminar wave area flows at very low speed in the upstream direction of the machine. When a piece arrives in contact with the laminar wave, a partial reversal of the alloy flow occurs and part of the alloy flows in the downstream direction of the machine.
The machines are therefore usually provided with what can be described as a weir system, whose height can be used to control the flow rate of the downstream flow of the solder. This weir system can consist simply of a metal plate or a guide chute for returning the solder to the surrounding bath.
One notes that the flow rate and the direction of flow of the alloy in this laminar wave area have a determining influence on the quality of the resulting soldering.
It must also be noted that some users, in order to adapt to the very specific characteristics of their production, substantially limit this downstream solder flow, preferring to establish a very slight or nearly zero downstream overflow of solder.
Wave soldering (or tinning) machines are usually open to the ambient air atmosphere. One problem encountered by the users of such machines is the formation of oxide layers (called dross) at the surface of the solder bath as a result of its exposure to air, resulting in a not insubstantial loss of solder and the need to regularly clean the bath. For example, a medium-size machine can give rise to the formation of more than a kilogram of dross per hour of operation.
Considering the specific case of the laminar wave, it is readily understood that a zero or excessively small downstream overflow of solder will represent a major disadvantage since the dross constantly forming on the flat surface of the wave cannot be effectively eliminated and thus is deposited on the piece with significant adverse effects on the quality of the resulting soldering or tinning.
Without requiring any further description, it will be readily comprehended that this dross-formation phenomenon, described here at length for the case of the flat surface of the laminar wave in a wave soldering machine, applies even more to the flat surface of a static bath.
Various technical solutions have been heretofore proposed for protecting the solder bath from oxidation by the surrounding air. These solutions can be schematically divided into the following three categories.
a) A first category of solutions consists of setting up a confined protection atmosphere, at least above the solder bath, but also sometimes in the rest of the machine. Thus, completely inertized machines have appeared, designed from the outset as a gas-tight tunnel, but there have also appeared more simply cowling or hood systems that can be used on existing conventional machines open to the ambient air to set up a nitrogen blanket at least in the area of the solder bath.
In this first category of solutions, the applicant of U.S. Pat. No. 5,161,727 has proposed an inertization apparatus comprising a set of cowlings defining, at least directly above the solder bath, an interior space separated from the surrounding atmosphere by gas-tight means, with systems of gas injection channels opening into the upper parts of the diffuser-equipped cowlings.
While the inertization apparatus described in the aforesaid document certainly represents a substantial improvement over the performance of the existing systems (particularly in terms of optimizing the compromise between the flow rate of the injected gas and the concentration of residual oxygen achieved above the solder bath), this system still represents relatively complicated and costly designs since it must be virtually custom-fitted to each type of wave soldering machine present on the market.
b) A second category of solutions involves setting up an unconfined protective atmosphere using injectors localized in proximity to the solder wave without closure of the space above the waves.
The devices taught in WO 93/11653 fall into this second category.
Taking into account their extremely localized configuration, it is considered difficult to control the quality of inertization afforded by these processes, necessitating in practice the use of two symmetrical injectors to successfully achieve a low oxygen level.
Furthermore, none of the documents in this category of solutions deal with and provide solutions for the specific problems posed by laminar waves.
c) The third category of solutions to the problem of dross formation employs the use, at the surface of the laminar wave, of a film of oil with a high covering power.
The oil protection systems have the traditional disadvantages associated with the use of oil (particularly in the presence of a heat source), which in particular include the presence of oil deposits on the board (necessitating the implementation of an often difficult and imperfect cleaning), the necessity for scheduling frequent periods for machine maintenance due to the accumulation of oil in the solder bath, and oil vapor emissions which certainly represent a nuisance for the environment, whether for people or equipment.
One object of the present invention is therefore to propose a wave soldering or tinning machine that can achieve a localized inertization (without requiring the use of a confinement system) of the flat surface of a laminar wave (as already indicated, the laminar wave exhibits very specific operating problems due to its extremely low flow) wherein the design of the machine makes it possible to very simply and economically obtain a very favorable compromise between the residual oxygen content achieved at the surface of the laminar wave and the flow rate of the gas employed in the area of this wave, wherein this flow rate, when necessary to conform to the economic specifications of particular user sites, can be below 10 m3/h and is preferably less than or equal to 5 m3/h.
Another object of the present invention is to propose conditions which make it possible to achieve a significant reduction in dross formation on the flat surface of the laminar wave of the machine.
Studies conducted by the applicant have shown that such results can be obtained by the use of a gas injector that is localized in a position adjacent to and downstream from the laminar wave and is provided with a wall facing this wave wherein this wall has at least one group of openings positioned thereon so as to generate a first gas jet directed toward the flat surface of the laminar wave.
These studies also demonstrated the advantageous features of the combined use of the following measures:
use of a weir system having, for example, the form of a plate or a guide chute, for the downstream spillover of the solder into the bath, wherein adjustment of the height of the weir system with respect to the wave permits adjustment of the flow rate of the laminar wave overflow in the forward direction (i.e., the downstream direction of the machine);
in the case of use of a plate, having this plate dip into the solder bath;
in the case of use of a chute, using a chute which dips into the bath, or equipping the chute with a skirt which dips into the bath, in order to extend its action to some degree; and
use of a gas injector located in a position adjacent to and downstream from the laminar wave and provided with a wall facing this wave, wherein this wall has at least two groups of openings that direct the gas toward the wave, with a first group of openings being positioned so as to generate a first gas jet directed toward the flat surface of the laminar wave, while a second group of openings is positioned on the wall so as to inject a second gas jet into the space located between the plate and the injector (in the case of a xe2x80x9cplatexe2x80x9d weir) or in the interior of the skirt (in the case of xe2x80x9cchutexe2x80x9d weir).
As developed in greater detail below in connection with examples, the combined use of these measures gave an extremely effective inertization, even in the difficult case of a very nearly stagnant laminar wave, without requiring any confinement means: it was thus demonstrated that, at the time of board passage, a residual oxygen content of only a few tens of ppm could be obtained using a very moderate gas delivery rate (of only a few m3/h).